This invention relates to the removal of organic pollutants from a fluid stream, and particularly to a structure for the photocatalytic oxidation of such pollutants and to the method of making such structure.
It is now relatively well known that a photocatalytic process may be used to cleanse a fluid stream, typically an air stream, of organic pollutants. A photocatalyst, such as titania (TiO2, titanium dioxide), may be supported on some form of skeletal, or support, structure to provide a catalyst matrix for photochemical activation and cleansing interaction with organic contaminants in the fluid stream. A source of ultraviolet illumination serves to activate the photocatalyst. This type of air purifier may find use in a number of applications, and particularly in an enclosed environment such as a room or the interior of a vehicle. Recent focus has been on the use of such purifiers in buildings, automobiles and aircraft.
Numerous formulations of photocatalysts, as well as the process for their application and adherence to supporting substrates, have been disclosed. Moreover, numerous configurations and materials have been disclosed and/or used for the support of photocatalyst material. Examples of such formulations and/or structures may be found in the following patent publications, including PCT published application WO 96/37280 for PCT Application US96/05103 assigned to the assignee of the present application, as well as U.S. Pat. Nos. 6,063,343; 5,993,738; 5,993,702; 5,854,169; 5,849,200; 5,835,840; 5,790;934; 5,616,532; 5,126,111; 5,032,241; 4,966,759; and 4,892,712.
Typically, the preferred photocatalyst is a coating of TiO2 on a supporting substrate of any of various materials having various geometries. In some instances, the supporting substrates are meshes of fiberglass or similar material, e.g., ceramic and metallic foams, whereas in others they are fins or honeycomb-type structures. With respect to mesh-type substrates, a large fraction of the supported catalyst is essentially inactive because the inherent structure of these materials obscures the necessary ultraviolet illumination. Moreover, there may be difficulty in obtaining a uniformity or consistency of flow characteristics from one unit to another because of the amorphous or random nature of fluid passages in meshes, particularly after being coated with a photocatalyst. Fin-type supporting structures may be arranged to provide more uniform characteristics, but are often complex in their manufacture and assembly. Moreover, some such fin or honeycomb-type supporting structures have been of paper or ceramics, but paper has the limitations of low structural integrity and relatively short lifetime, and many of the ceramics may have a relatively high manufacturing cost, excessive weight and low mechanical durability due to brittleness. Moreover, the materials used in some such supporting structures and/or their assembly may give rise to the release of objectionable volatile organic compounds (VOCs) during operation.
In addition to the considerations about the suitability of the structural support, there is the further concern regarding the formulation and application of the photocatalyst to the selected structural support. The photocatalyst coating must posses the desired catalytic properties, while also being relatively economical to formulate, apply and continue to use. In this latter regard, it is important that the photocatalyst form a durable bond with the supporting substrate to prevent transfer of catalyst to the fluid stream being processed.
Accordingly, it is an object of the invention to provide a structural support for a photocatalyst which is of relatively low cost, light weight, high structural integrity, possesses good flow characteristics, and can be illuminated effectively by an external ultraviolet source.
It is a further object of the invention to provide a photocatalytic coating for such structural support which is adherent, durable, and possesses good photocatalytic properties.
It is a still further object of the invention to provide a photocatalyst and support therefore, which are not themselves a source of VOCs during operation.
The present invention provides an improved honeycomb photocatalyst matrix for purifying fluid flowing therethrough. The honeycomb photocatalyst matrix comprises a honeycomb-shaped skeletal structure of lightweight material, possibly a ceramic or more typically a metal, such as aluminum, formed or assembled as an array of multiple, substantially parallel cells open at opposite ends for fluid flow therethrough and illumination therewithin, the cells having respective surfaces, and a coating of photocatalyst on the surfaces of the cells, whereby the coated surfaces are activated by ultraviolet illumination to remove contaminants from fluid flowing through the cells.
The honeycomb-shaped skeletal structure of one embodiment is formed of a plurality of metal sheets, each having an alternating series of crests and troughs, and the sheets are stacked such that the crests and troughs form the respective cells. Planar metal parting sheets may be interposed between the sheets having troughs and crests. The metal is preferably aluminum, and may be about 0.002 inch in thickness.
In another embodiment, the metal sheets which define the cells are planar, and a portion of the sheets are oriented orthogonally to and interfitted with the remainder of the sheets, in substantially eggcrate fashion, to define the cells.
A frame is included about the outer perimeter of the honeycomb-shaped skeletal structure to maintain stability transverse to fluid flow, and one or more stabilizer members joined with the frame serve to resist displacement of the structure relative to the frame in the direction of fluid flow. The stabilizer members may be adjacent to one, or both, end faces of the cell structure, or may extend therethrough. The entire structural assembly is made without including volatile organic compounds (VOCs) as structural or bonding materials, since VOCs degrade when exposed to ultraviolet light.
A durable, adherent, photocatalytic coating is provided on the substrate material via the use of a ceramic interface material to promote adhesion of the photocatalyst. Although the substrate may itself be a, or the, ceramic material that provides such interface, the use of a thin metal sheet substrate (such as aluminum) having a ceramic layer formed thereon is preferred. Prior to coating, the aluminum sheets are provided with a ceramic oxide layer on one or both surfaces. The oxide layer may be grown or native. Titania powder is then mixed in a TiO2 sol-gel solution to form a titania slurry. The oxidized surfaces of the aluminum substrate are coated with the titania slurry, and the coated substrate is then heat treated to calcine, harden, and bond the titania coating to the substrate.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.